If it is desired, for instance, to incubate living cells in a medium over a longer period it is important to be capable of adjusting the atmosphere parameters in the incubation chamber with exactness. Cells are, for instance, frequently cultivated in small containers that are filled with an aqueous medium. An evaporation of the aqueous medium results in changed concentrations in the medium which, again, may have the result that the cells die off. Therefore, in order to prevent this evaporation and desiccation, there is a need for the ability to control the humidity, in particular to adjust a high relative gas humidity, respectively air humidity.
In the prior art various methods are known to achieve such a humidity control.
According to a first alternative atomizers are applied which inject atomized liquid into the gas volume. The small water drops are absorbed by the gas which is unsaturated with water. The gas humidity is controlled by the amount of injected liquid. A reduction of the gas humidity can only be obtained by the gas exchange to the environment.
It is a drawback of an atomizer, however, that the gas mixture at the nozzle outlet of this atomizer is supersaturated, which may lead to a condensation and corrosion. Moreover, the humidity of the gas volume has to be less than 100% to allow the absorption of the injected amount of water. Consequently, portions of the gas volume have relative humidities of less than 100%, and the gas in the proximity of the nozzle is supersaturated. Such gradients in the gas volume are not desirable, however. In order to reduce the humidity it is furthermore required to open the incubation chamber. Eventually, the temperature of the gas at the atomizer falls to a great extent owing to the evaporation energy, and this leads to undesired temperature gradients in the gas, too.
One alternative solution is an evaporator, which conventionally comprises a container with water which is mounted in the gas volume. The liquid in the container evaporates and humidifies the gas until an equilibrium has been obtained. This equilibrium corresponds to the saturation concentration. As long as the amount of water is enough and the water surface is correspondingly large it will always be possible to obtain an equilibrium in this way.
A disadvantage is the inertia of this system, however, because the system requires a certain period of time until an equilibrium is established. Although the evaporation of the liquid can be accelerated by increasing the temperature, this process, too, is not very fast. If a system is used that requires a gas exchange, achieving the saturation concentration is no longer ensured. Finally, also the open water surface in the gas volume represents a drawback, e.g. in moving applications or applications replete with vibrations.
Finally, it is also known to use gas washing bottles for humidifying the gas. In this case, dry gas is conducted through a liquid so as to humidify the gas. This humidification happens fast as gas bubbles form a large surface with the water. The humid gas mixture can than be conducted into the occluded gas volume of the incubation chamber. The relative humidity in the gas volume in the incubation chamber can then be controlled by the liquid absorbed by and transported in the gas. The amount of liquid absorbed by the gas can be controlled by the water bath temperature as the absorption capacity of the gas is temperature-dependent. A reduction of the humidity, again, requires a gas exchange in the volume, i.e. the incubation chamber has to be opened.
This system allows a fast humidification of the gas. To allow the controlling of the humidity in a small gas flow it is necessary, however, that plenty of humidity is transported in the inflowing gas. This is usually accomplished by adjusting the temperature of the liquid bath through which the dry gas is conducted. Thus, the inflowing gas is heated, and the absorption of liquid is increased. Many liquids, e.g. water, have a high heat capacity, however, so that the system becomes quite inert as a fast temperature adjustment is impossible. Thus, it is impossible to control the humidity of the gas volume in the incubation chamber fast enough, and this frequently entails condensation problems if the gas volume is operated near the saturation concentration. Furthermore, it is impossible to go below a certain minimum humidity as the inflowing gas is humidified even if the liquid bath of the gas washing bottle is not heated. The control range is typically between 60% and 100% of the relative gas humidity.